Again: word.

I just think - at least on Deans' part - it expresses a sort of "familiar-ness"; a playfull, bantery aspect of the otherwise mutually respectfull relationship betweem Castiel and Dean.
As for the others... I can kinda get why Anna would have the right to call him Cas - they were "involved" (whatever that means in Angel World) and she should be able to express that. Still, I find myself hating it >_<

Give the lil'one a hug from me :D